Over the past 15 years there have been a number of instances where plastics have replaced metal engine components. Examples include valve covers and oil pans. These applications met with varying degrees of success, depending on the suitability of the material. Several thermoplastic valve covers and an oil pan application were terminated due to oil leakage resulting from distortion or cracking. In the past few years, thermoset composites have demonstrated excellent performance in commercial valve cover applications and are scheduled for introduction in future under-the-hood programs. This paper compares properties and relative costs of several thermoplastic and thermoset materials offered for automotive engine applications. Properties such as rigidity at elevated temperature, flammability, effects of exposure to heat and automotive fluids, and recyclability are discussed.